The increasing availability of reasonably priced satellite communication services, and a variety of narrow bandwidth (voice/data) and wide bandwidth (video) devices to meet the needs of a broad spectrum of communication system users has led to communication system architectures that can be tailored in terms of connectivity structure and customer utilization. This diversity of equipment types and signal processing capability has resulted in the desirability of having `local` area networks (LANs), customarily limited to terrestrial-based systems of a limited geographical area, be expanded to encompass a much larger scale of communication services, preferably those employing satellite link transmission equipment to connect terminal devices among well dispersed office sites.
To facilitate inter-office communications, it is preferred to have such satellite-based systems configured as mesh networks. A simplified example of such a network is shown diagrammatically in FIG. 1 as being comprised of four earth stations 11, 12, 13, 14. In such a network, for interstation connectivity, each terminal device 10 has a direct satellite link 20 (via one hop through a relay satellite 30) to any other terminal device 10 in the network. Connectivity between a respective terminal device 10 that is ported to an associated earth station interface and a respective terminal device 10 that is ported to another earth station interface may be effected by providing each earth station with a multiplexing, demultiplexing subsystem, that is operative to controllably uplink messages from any terminal device (e.g. audio (voice), data, video equipment) over an outbound link and to distribute downlink messages to their intended destination terminal devices.
One type of multiplexing scheme involves a time division multiplexing (TDM) and demultiplexing arrangement, through which a fixed number of bytes for each user port are allocated within a fixed information frame. The frame size (total number of bytes) may be determined by the number of ports and their data rates, and the number of frames transmitted per second. The number of TDM frames per second determines the aggregate data rate. The aggregate data rate includes the total user port data rate plus framing overhead.
Interfacing respective terminal devices with the TDM subsystem may be effected by means of a dedicated multiport switch associated with the respective multiplexer and demultiplexer units of the earth station, with each multiport switch being configured for an equal number of data communication equipment (DCE) and data terminal equipment (DTE) ports, so as to provide full matrix capability between DCE and DTE ports. The port speed and format (DCE to DTE) must match; however, matrix switches can usually translate between different physical and electrical characteristics.
A problem associated with such a TDM-matrix switch earth station architecture proposal is the fact that its terminal-to-terminal connectivity involves dedicated port connections, which remain fixed unless the system is physically reconfigured. As a result, in such a system, only a very limited selectivity for voice calls is afforded, since only point-to-point connections can be effected between voice multiplexers and not among the voice circuits themselves that connect to the voice multiplexers. In addition, TDM schemes are very sensitive to timing and network synchronization, since no queuing is performed. A master network timing source is required for all network subsystems. Also, because suppliers of multiplexer and matrix switch components are not the same, different monitor and control mechanisms are necessary for each respective piece of equipment. This need is further burdened by the fact that, due to the unique character of a simplex data stream, the required multiplexer/demultiplexer is not an off-the-shelf product. Finally, the cost of such a system is not insignificant, as each of the multiport switch and the multiplexer and demultiplexer components must be purchased separately.
To solve this problem, we have developed a satellite communication network, which employs an earth station architecture having a frame relay protocol-based multiplex switching mechanism to control the uplink and downlink routing of messages among the terminals in the network. This frame relay protocol-based earth station architecture is described in the U.S. Pat. No. 5,434,850, by D. Fielding et al, entitled: "FRAME RELAY PROTOCOL-BASED MULTIPLEX SWITCHING SCHEME FOR SATELLITE MESH NETWORK," issued Jul. 18, 1995, assigned to the assignee of the present application and the disclosure of which is incorporated herein.
In accordance with the above-referenced patented invention, a network interface `frame relay` standard defines the multiplexing of multiple virtual ports across individual physical communication port. The interface standard `frame relay` is based upon the transmission and reception of individual frames or packets of information serially through a port, with a respective frame of digital data containing additional address and control bytes for routing and elementary error detection and flow control.
The frame relay switch is ported via a first set of terminal ports to a plurality of `local` terminal devices, which may include respective voice, data and video signalling equipments. A voice signal link transports low bit rate digitized voice signals (e.g., those having an encoding rate of less than 10 kb/s), to and from a voice signal multiplexer, in order to interface voice traffic with a plurality of voice signalling circuits that are selectively accessible through the multiplexer. The voice signalling link also conveys call supervision signals, including dial tone detection, dialing, circuit busy, call connect and call termination control and status signals. The voice signal multiplexer is operative to append and decode terminal device selectivity information to the address field portion of a frame processed by the frame relay switch. Also ported to the frame relay switch are one or more data links that may be coupled to two-way synchronous data terminal devices. An additional port of the frame relay switch may be coupled to a link for wide bandwidth signals, such as a video teleconferencing terminal.
On its satellite link side, the frame relay switch is coupled via a modem unit to an attendant RF transceiver unit having an associated satellite antenna unit for transmitting uplink channel signals to the relay satellite and receiving downlink channel signals from the satellite. The modem unit contains a single uplink modulator having a fixed transmission frequency associated with that earth station, and a plurality of downlink demodulators, which are respectively tuned to the transmission frequencies employed by each of the other stations of the network, thereby providing connectivity among the multiple earth stations of the network.
Using address and control fields employed by frame relay connectivity control software, the frame relay switch is dynamically configurable to provide multilayer addressing and device selectivity (filtering), thereby enabling point-to-point connectivity of multiple terminal devices, such as a plurality of voice circuits served by the voice circuit multiplexer unit to which a voice signal port of the frame relay switch is coupled. Dial codes on the trunk or station side of the voice signal link are translated into frame relay addresses (data link connection identifiers) that are added to each frame of data for routing through the network. With this additional layer of routing information, voice connectivity is available between any two voice terminal devices (e.g. trunks) in the network.
Now although the frame relay switch-based earth station architecture described in the above-referenced patent is a very effective scheme for providing connectivity among a reasonably small number of earth stations (e.g., eight to ten or less as a non-limiting example), which are generally fully utilized for terminal-to-terminal communications, simply providing a linear expansion of the system for the purpose of serving the needs of a less than majority of the time or occasional user is not a cost effective application of system resources. On the other hand, the less than full time user cannot be ignored. The problem is how to offer the communication functionality of a frame relay switch-based earth station to a reduced usage customer in a cost effective manner.